Magnetic tunnel junction (MTJ) to reduce spin transfer magnetization switching current

ABSTRACT

A MTJ that minimizes spin-transfer magnetization switching current (Jc) in a Spin-RAM to &lt;1×10 6  A/cm 2  is disclosed. The MTJ has a Co 60 Fe 20 B 20 /MgO/Co 60 Fe 20 B 20  configuration where the CoFeB AP1 pinned and free layers are amorphous and the crystalline MgO tunnel barrier is formed by a ROX or NOX process. The capping layer preferably is a Hf/Ru composite where the lower Hf layer serves as an excellent oxygen getter material to reduce the magnetic “dead layer” at the free layer/capping layer interface and thereby increase dR/R, and lower He and Jc. The annealing temperature is lowered to about 280° C. to give a smoother CoFeB/MgO interface and a smaller offset field than with a 350° C. annealing. In a second embodiment, the AP1 layer has a CoFeB/CoFe configuration wherein the lower CoFeB layer is amorphous and the upper CoFe layer is crystalline to further improve dR/R and lower RA to ≦10 ohm/μm 2 .

RELATED PATENT APPLICATIONS

This application is related to the following: Ser. No. 11/496,691,filing date Jul. 31, 2006; and Ser. No. 11/317,388, filing date Dec. 22,2005, assigned to a common assignee.

FIELD OF THE INVENTION

The invention relates to a high performance Magnetic Tunneling Junction(MTJ) element and a method for making the same, and more particularly,to a configuration comprised of a composite AP1 pinned layer, a MgObarrier layer, an amorphous CoFeB free layer, and a Hf capping layerthat minimizes the “dead layer” at the free layer/capping layerinterface and reduces spin transfer magnetization switching current in aSpin-RAM device.

BACKGROUND OF THE INVENTION

Magnetoresistive Random Access Memory (MRAM), based on the integrationof silicon CMOS with MTJ technology, is a major emerging technology thatis highly competitive with existing semiconductor memories such as SRAM,DRAM, Flash, etc. A MRAM device is generally comprised of an array ofparallel first conductive lines on a horizontal plane, an array ofparallel second conductive lines on a second horizontal plane spacedabove and formed in a direction perpendicular to the first conductivelines, and an MTJ element interposed between a first conductive line anda second conductive line at each crossover location. A first conductiveline may be a word line while a second conductive line is a bit line orvice versa. Alternatively, a first conductive line may be a bottomelectrode that is a sectioned line while a second conductive line is abit line (or word line). There are typically other devices includingtransistors and diodes below the array of first conductive lines as wellas peripheral circuits used to select certain MRAM cells within the MRAMarray for read or write operations.

An MTJ element may be based on a tunneling magneto-resistance (TMR)effect wherein a stack of layers has a configuration in which twoferromagnetic layers are separated by a thin non-magnetic dielectriclayer. In an MRAM device, the MTJ element is formed between a bottomelectrode such as a first conductive line and a top electrode which is asecond conductive line. An MTJ stack of layers that are subsequentlypatterned to form an MTJ element may be formed in a so-called bottomspin valve configuration by sequentially depositing a seed layer, ananti-ferromagnetic (AFM) pinning layer, a ferromagnetic “pinned” layer,a thin tunnel barrier layer, a ferromagnetic “free” layer, and a cappinglayer. The AFM layer holds the magnetic moment of the pinned layer in afixed direction. In a MRAM MTJ, the free layer is preferably made ofNiFe because of its reproducible and reliable switching characteristicsas demonstrated by a low switching field (Hc) and switching fielduniformity (σHc). Alternatively, an MTJ stack may have a top spin valveconfiguration in which a free layer is formed on a seed layer followedby sequentially forming a tunnel barrier layer, a pinned layer, AFMlayer, and a capping layer.

The pinned layer has a magnetic moment that is fixed in the “y”direction, for example, by exchange coupling with the adjacent AFM layerthat is also magnetized in the “y” direction. The free layer has amagnetic moment that is either parallel or anti-parallel to the magneticmoment in the pinned layer. The tunnel barrier layer is thin enough thata current through it can be established by quantum mechanical tunnelingof conduction electrons. The magnetic moment of the free layer maychange in response to external magnetic fields and it is the relativeorientation of the magnetic moments between the free and pinned layersthat determines the tunneling current and therefore the resistance ofthe tunneling junction. When a sense current is passed from the topelectrode to the bottom electrode in a direction perpendicular to theMTJ layers, a lower resistance is detected when the magnetizationdirections of the free and pinned layers are in a parallel state (“1”memory state) and a higher resistance is noted when they are in ananti-parallel state or “0” memory state.

In a read operation, the information stored in an MRAM cell is read bysensing the magnetic state (resistance level) of the MTJ element througha sense current flowing top to bottom through the cell in a currentperpendicular to plane (CPP) configuration. During a write operation,information is written to the MRAM cell by changing the magnetic statein the free layer to an appropriate one by generating external magneticfields as a result of applying bit line and word line currents in twocrossing conductive lines, either above or below the MTJ element. Incertain MRAM architectures, the top electrode or the bottom electrodeparticipates in both read and write operations.

A high performance MTJ element is characterized by a highmagnetoresistive (MR) ratio which is dR/R where R is the minimumresistance of the MTJ element and dR is the change in resistanceobserved by changing the magnetic state of the free layer. A high MRratio of over 130% and a low magnetostriction (λ_(s)) value of about1×10⁻⁶ or less are desirable for Spin-RAM applications. This result isaccomplished by (a) well controlled magnetization and switching of thefree layer, (b) well controlled magnetization of a pinned layer that hasa large exchange field and high thermal stability and, (c) integrity ofthe tunnel barrier layer. In order to achieve good barrier propertiessuch as a specific junction resistance x area (RA) value and a highbreakdown voltage (Vb), it is necessary to have a uniform tunnel barrierlayer which is free of pinholes that is promoted by a smooth and denselypacked growth in the AFM and pinned layers. Although a high RA value ofabout 10000 ohm-μm² is acceptable for a large area (A), RA should berelatively small (<1000 ohm-μm²) for smaller areas. Otherwise, R wouldbe too high to match the resistance of the transistor which is connectedto the MTJ.

Generally, the purpose of the capping layer is to protect underlyinglayers in the MTJ during etching and other process steps and to functionas an electrical contact to an overlying conductive line. The typicalcapping layer for an MTJ stack is a non-magnetic conductive metal suchas Ta or TaN. During thermal annealing, Ta is capable of getteringoxygen atoms originating in the NiFe free layer. Consequently, the NiFefree layer is less oxygen contaminated and a more distinct boundarybetween the tunnel barrier layer and NiFe free layer is thereby obtainedto improve dR/R. The disadvantage of using a Ta capping layer is that Tadiffuses into NiFe during thermal annealing, especially at highannealing temperatures (i.e. >250° C.) to produce an alloy that not onlyreduces free layer moment (Bs) but makes NiFe very magnetostrictive witha λ_(s) of ≧5×10⁻⁶. Thus, alternative capping layer materials aredesirable that minimize inter-diffusion between a free layer and cappinglayer, serve as a good oxygen getter material, and enable both a high MRratio and low λ_(s) value to be achieved in MTJs for advanced MRAM andTMR read head technologies.

Spin transfer (spin torque) magnetization switching as described by J.Sloneczewski in “Current-driven excitation of magnetic multilayers”, J.Magn. Materials V 159, L1-L7 (1996), and by L. Berger in “Emission ofspin waves by a magnetic multiplayer traversed by a current” in Phys.Rev. Lett. B, Vol. 52, p 9353 has stimulated considerable interest inrecent years due to its potential application for spintronic devicessuch as MRAM on a gigabit scale. The spin-transfer effect arises fromthe spin dependent electron transport properties offerromagnetic-spacer-ferromagnetic multilayers. When a spin-polarizedcurrent transverses a magnetic multilayer in a CPP configuration, thespin angular moment of electrons incident on a ferromagnetic layerinteracts with magnetic moments of the ferromagnetic layer near theinterface between the ferromagnetic and non-magnetic spacer. Throughthis interaction, the electrons transfer a portion of their angularmomentum to the ferromagnetic layer. As a result, spin-polarized currentcan switch the magnetization direction of the ferromagnetic layer if thecurrent density is sufficiently high, and if the dimensions of themultilayer are small. The difference between a Spin-RAM and aconventional MRAM is only in the write operation mechanism. The readmechanism is the same.

Referring to FIG. 1, a memory cell 1 of a Spin-RAM includes a MTJ 13,word line (WL) 6, bit line (BL) 14, bottom electrode 7, and a CMOStransistor having a source 3, drain 4, and p-type semiconductor 2, forexample, that provides current for switching the MTJ free layer 11.There is also a gate electrode 5. Additional layers in the MTJ 13 are anAFM layer 8, pinned layer 9, insulating barrier 10, and capping layer12.

A critical current for spin transfer switching (Ic), which is defined as[(Ic⁺+|Ic⁻|)/2], for the present 180 nm node sub-micron MTJ having atop-down area of about 0.2×0.4 micron, is generally a few milliamperes.The critical current density (Jc), for example (Ic/A), is on the orderof several 10⁷ A/cm². This high current density, which is required toinduce the spin-transfer effect, could destroy a thin insulating barrier10 such as AlOx, MgO, or the like. In order for spin-transfermagnetization switching to be viable in the 90 nm technology node andbeyond, the critical current density (Jc) must be lower than 10⁶ A/cm²to be driven by a CMOS transistor that can typically deliver 100 μA per100 nm gate width. For Spin-RAM applications, the (ultra-small) MTJsmust exhibit a high tunnel magnetoresistance ratio (TMR or dR/R) muchhigher than the conventional MRAM-MTJ that use AlOx as a barrier layerand have a dR/R of about 40% as stated by Z. Diao et. al in “Spintransfer switching and spin polarization in MTJ with MgO and AlOxbarrier”, Appl. Phys. Lett, 87, 232502 (2005). D. Djayaprawira et. al in“230% room temperature magnetoresistance in CoFeB/MgO/CoFeB MTJ”, Appl.Phys. Lett. V 86, p. 092502 (2005) demonstrated that a highly oriented(001) CoFeB/MgO/CoFeB MTJ is capable of delivering dR/R>200%. Therefore,it is essential to find a way to combine a high TMR ratio of aCoFeB/MgO/CoFeB MTJ and the current driven switching capabilitynecessary to make Spin-RAM a practical technology.

To apply spin-transfer switching to MRAM technology, it is desirable todecrease Ic (and its Jc) by more than an order of magnitude so as toavoid an electrical breakdown of the MTJ device and to be compatiblewith the underlying CMOS transistor that is used to provide switchingcurrent and to select a memory cell. A means to improve the dielectricbreakdown voltage is also an important consideration.

The intrinsic critical current density (Jc) as given by Slonczewski ofIBM is shown in equation (1) below.Jc=2eαMst _(F)(Ha+H _(k)+2πMs)/

η  (1)where e is the electron charge, α is a Gilbert damping constant, t_(F)is the thickness of the free layer,

 is the reduced Plank's constant, η is the spin-transfer efficiencywhich is related to the spin polarization (P), Ha is the externalapplied field, and H_(k) and Ms are respectively, uniaxial anisotropyand magnetization of the free layer.

Normally, the demagnetizing field, 2πMs (several thousand Oe term) ismuch larger than the uniaxial anisotropy field Hk and external appliedfield (approximately 100 Oe) Ha term, hence the effect of Hk and Ha onJc are small. In equation (2), V equals Ms(t_(F)A) and is the magneticvolume which is related to the thermal stability function termK_(u)V/k_(b)T where K_(u) is the magnetic anisotropy energy and k_(b) isthe Bolzmann constant.Jc∝αMsV/P  (2)

A routine search of the prior art was conducted and the followingreferences were found. Hosomi et al. in “A novel non-volatile memorywith spin torque transfer magnetization switching: Spin-RAM”, 2005 IEDM,paper 19-1, present a Spin-RAM with spin-torque transfer magnetizationswitching for the first time. Sony's Spin-RAM devices were fabricatedwith a Co₄₀Fe₄₀B₂₀/RF sputtered MgO/Co₄₀Fe₄₀B₂₀ (pinned layer/tunnelbarrier/free layer) MTJ configuration that was processed with a 350°C.-10K Oe annealing. The CoFeB/MgO/CoFeB MTJ is employed for its highpolarization (P) that provides a high output signal for TMR. MTJ size is100 nm×150 nm with an oval shape. A tunnel barrier layer is made ofcrystallized (100) MgO whose thickness is controlled to <10 Angstromsfor the proper RA of about 20 ohm-μm² while dR/R or TMR (intrinsic) ofthe MTJ is 160%. Using a 10 ns pulse width, the critical currentdensity, Jc, for spin transfer magnetization switching is about 2.5×10⁶A/cm² which means Ic is equal to 375 μA. Due to a very small MTJ size,resistance distribution of Rp (low resistance state) and Rap (highresistance state) has a sigma (Rp_cov) around 4%. Thus, for a readoperation, TMR(without bias)/Rp_cov=40 and this ratio is equivalent tothat for a conventional CoFeB/AlOx/NiFe (pinned layer/tunnelbarrier/free layer) MRAM MTJ configuration in which TMR is typically 40%with an Rp_cov of around 1%. Note that MTJ size in this case is 300nm×600 nm. In addition, a CoFeB/AlOx/NiFe MRAM MTJ in a read operationis typically 300-350 mV biased. Under this condition, TMR(350 mVbias)/Rp_cov would be reduced to about 20.

A spin transfer magnetization switching of a Co₆₀Fe₂₀B₂₀/MgO/Co₆₀Fe₂₀B₂₀MTJ is reported by Y. Huai et al. in “Spin transfer switching currentreduction in magnetic tunnel junction based dual filter structures” inAppl. Physics Lett., V 87, p. 222,510 (2005). The nominal MTJ size is125 nm×220 nm with an RA of about 50 ohm-μm² and dR/R=155%. Jc₀ (i.e. Jcextrapolated to a pulse width of 1 ns) is ˜2×10⁶ A/cm², similar to theSony example. For a dual spin filter (DSF) structure wherein free layerswitching is affected by two spin torques, Jc has been reduced to˜1.3×10⁶ A/cm².

In another reference by J. Hayakawa et al. entitled “Current-drivenmagnetization switching in CoFeB/MgO/CoFeB magnetic tunnel junctions”,Japan J. Appl. Phys. V 44, p. 1267 (2005), a Jc (at a 10 ns pulse width)is reported as 7.8×10⁵, 8.8×10⁵, and 2.5×10⁶ A/cm² for MTJs processedwith 270° C., 300° C., and 350° C. annealing temperatures, respectively.RA for the MTJ that has an 8.5 Angstrom MgO tunnel barrier thickness isabout 10 ohm-μm². TMR (intrinsic) or dR/R ratios as a function of thethree annealing temperatures for the Co₄₀Fe₄₀B₂₀/MgO/Co₄₀Fe₄₀B₂₀ MTJswith a 20 Angstrom thick Co₄₀Fe₄₀B₂₀ free layer are 49%, 73%, and 160%,respectively. It was found that a CoFeB free layer which is annealed at270° C. or 320° C. is amorphous as described by S. Cardoso et. al in“Characterization of CoFeB electrodes for tunnel junction”, J. Appl.Phys., V 97, p. 100916 (2005). On the other hand, a CoFeB free layerannealed at 350° C. is crystalline. It has been confirmed that thedamping constant for an amorphous CoFeB layer is about half that of acrystalline CoFeB layer by C. Bilzer et. al in “Study of the dynamicmagnetic properties of soft CoFeB films”, J. Appl. Phys., V 100, p.053903 (2006). Amorphous layers showed a low damping (α=0.006) that isthickness dependent while crystalline CoFe with no B content has a value2× higher (α=0.013).

The aforementioned references can be summarized with the following fourpoints: (1) Jc is greater than 2×10⁶ A/cm² for a CoFeB₂₀/MgO/CoFeB₂₀ MTJhaving a crystalline free layer; (2) a Jc less than 1.0×10⁶ A/cm² can beachieved for a MTJ with an amorphous CoFeB free layer although dR/R isless than 100%; (3) a TMR(with bias)/Rp_cov (4%)≧20 is required for aCoFeB₂₀/MgO/CoFeB₂₀ MTJ to be useful in a Spin-RAM application; and (4)Jc can be reduced in half by employing a dual spin filter (DSF) MTJstructure. However, to our knowledge, none of the prior art referenceshas achieved a Jc less than 1.0×10⁶ A/cm² and a dR/R over 120% togetherwith a TMR(300 mV bias)/Rp_cov (4%) of at least 20 which is believed tobe necessary for a Spin-RAM application. Furthermore, it is believedthat the DSF MTJ structure may be too difficult to manufacture and thatit is more desirable to fabricate a Spin-RAM device using a MTJ having asingle spin valve structure.

In other prior art references, U.S. Pat. No. 6,831,312 discloses a listof amorphous alloys such as CoFeB and CoFeHf. Crystallizationtemperature for CoFeHf is >350° C. and for CoFeB is 325° C.-350° C. Anamorphous free layer of CoFeB is also described in U.S. PatentApplication 2005/0277206 and in U.S. Patent Application 2006/0003185.U.S. Pat. No. 6,990,014 teaches that an information recording layercomprises amorphous CoFeB. U.S. Patent Application 2006/0209590discloses an amorphous CoFeB layer over a MgO tunnel barrier layer.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a MTJ element for aSpin-RAM device that has a configuration which is capable of achieving aJc<1.0×10⁶ A/cm² and a TMR(300 mV bias)/Rp_cov (4%) of at least 20.

A second objective of the present invention is to provide a MTJ elementhaving a single spin valve structure that satisfies the first objective.

A third objective of the present invention is to provide a MTJ elementaccording to the first two objectives that produces an RA of about 10-20Ω/μm² and a dR/R of greater than 120% that is suitable for spin-transfermagnetization switching applications.

According to a first embodiment, these objectives are achieved byproviding a substrate comprised of a bottom conductor electrode on whicha Spin-RAM structure is to be fabricated. An MTJ element is formed byfirst depositing a stack of layers on the bottom conductor electrode. Inone aspect, the MTJ stack has a bottom spin valve configuration in whicha seed layer, AFM layer, synthetic anti-ferromagnetic (SyAF) pinnedlayer, tunnel barrier layer, free layer, and a capping layer aresequentially formed. Preferably, the pinned layer has a syntheticanti-ferromagnetic (SyAF) configuration wherein a Ru coupling layer issandwiched between a lower CoFe (AP2) layer and an upper amorphous CoFeB(AP1) layer. The tunnel barrier layer may be comprised of crystallineMgO. Above the tunnel barrier layer is a free layer comprised ofamorphous CoFeB. A Hf/Ru capping layer may be employed on the freelayer. The lower layer in the capping layer is preferably Hf in order toreduce the magnetic “dead layer” at the free layer/capping layerinterface. All of the layers in the MTJ stack are formed by sputteringor ion beam deposition (IBD). The MgO tunnel barrier layer is typicallyformed by depositing a first Mg layer, oxidizing the metal by a naturaloxidation (NOX) or radical oxidation (ROX) method, and then depositing asecond Mg layer on the oxidized first Mg layer. The MTJ isadvantageously annealed at a temperature in the range of about 250° C.to 300° C. to ensure an amorphous character in the CoFeB pinned and freelayers. Once all the layers in the stack are laid down and thermallyannealed to fix the pinned layer magnetization direction, a conventionalpatterning and etching sequence is followed to fabricate a MTJ element.Thereafter, a dielectric layer is typically deposited on the substrateand MTJ, and is thinned to be coplanar with the capping layer. A topconductor may then be formed on the MTJ and dielectric layer.

In a second embodiment, the AP1 layer in the MTJ stack is modified byadding a crystalline CoFe layer on the amorphous CoFeB layer such thatthe CoFe layer contacts the MgO tunnel barrier. A Co₆₀Fe₂₀B₂₀ lower AP1layer may be annealed at 265° C., for example, to form an amorphouslayer that provides a smooth surface for growing smooth MTJ layersthereon. The CoFe upper AP1 layer preferably has a Co₇₅Fe₂₅ content witha body centered cubic (bcc) structure that is advantageously employed togrow an overlying (100) MgO tunnel barrier. The crystalline MgO tunnelbarrier is formed by a ROX or NOX process as in the first embodiment.The capping layer may be Ta, Hf, or Zr.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a conventional memory cell in aSpin-RAM device.

FIG. 2 is cross-sectional view of a partially formed Spin-RAM that hasan MTJ structure according to one embodiment of the present invention.

FIG. 3 is a cross-sectional view of the Spin-RAM structure in FIG. 2after a photoresist mask is removed and an insulation layer is formedadjacent to the MTJ element and a bit line is formed on the top surfaceof the MTJ element.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a MTJ configuration that provides a uniquecombination of a low Jc, low RA value, and high dR/R that is suitablefor spin-transfer magnetization switching applications. Devices based onthis technology may be referred to as Spin-RAM or STT-RAM devices.Drawings are provided by way of example and are not intended to limitthe scope of the invention. Further, the drawings are not necessarilydrawn to scale and the relative sizes of various elements may differfrom those in an actual device.

A Spin-RAM structure formed according to a first embodiment of thepresent invention will now be described. Referring to FIG. 2, apartially completed Spin-RAM structure 40 is shown that includes asubstrate 20 which may be silicon or another semiconductor substrateused in the art that typically contains devices such as transistors anddiodes. A first insulation layer 22 comprised of Al₂O₃, silicon oxide,or the like is disposed on the substrate 20. There is a first conductiveline comprised of copper, for example, formed within and coplanar withthe first insulation layer 22. For the purpose of this discussion, thefirst conductive line is a word line 21 that is electrically connectedto the transistor source electrode (not shown). Optionally, the firstconductive line may be called a digit line, data line, row line, orcolumn line as appreciated by those skilled in the art. Note that unlikeconventional MRAM, magnetization switching in the Spin-RAM MTJ isaccomplished by passing current through the MTJ and not by fieldsinduced by current in the word line and bit line.

There is a second insulation layer 23 such as Al₂O₃ or silicon oxideformed on the word line 21 and first insulation layer 22. Above thesecond insulation layer 23 is a bottom conductor layer 24 that isinterconnected to an underlying transistor (not shown) in substrate 20.The bottom conductor layer 24 may be a composite layer comprised of alower seed layer, middle conductive layer, and upper capping layer (notshown). Furthermore, word line 21 and bottom conductor layer 24 may beconnected to a source and drain of a transistor element, respectively,similar to the configuration depicted in FIG. 1 for word line 6 andbottom electrode 7.

It should be understood that the Spin-RAM structure is part of an arrayin which multiple parallel word lines are formed in a first conductivelayer and multiple top conductor electrodes such as parallel bit linesare formed in a second conductive layer above an array of MTJs.Alternatively, the first conductive layer may be parallel bit lineswhile the second conductive layer is parallel word lines. The word linesand bit lines are aligned orthogonal to each other and a bottomconductor layer may be used to connect each MTJ element with atransistor in the substrate. In the exemplary embodiment, an MTJ elementis formed between a bottom conductor layer and bit line at each locationwhere a bit line crosses over a word line.

The bottom conductor layer 24 may be a sectioned line, for example, thathas a rectangular shape in the x, y plane and a thickness in the zdirection. Alternatively, the bottom conductor layer 24 may be a bitline that is aligned orthogonally to an underlying word line 21 and to asubsequently formed second word line above the MTJ. In one embodimentwhere the bottom conductor layer has a seed layer/conductivelayer/capping layer configuration, the seed layer may be comprised ofNiCr, Ta, or TaN. The conductive layer may be made of Ru, Rh, Ir orother metals such as Au, Cu, or α-Ta. The capping layer may be anamorphous Ta layer, for example, that serves to promote uniform anddense growth in subsequently formed MTJ layers.

An MTJ stack of layers is now formed on the bottom conductor layer 24.It should be understood that the MTJ stack may be formed in the sameprocess tool as the bottom conductor layer. For instance, the bottomconductor layer 24 and MTJ stack may be formed in an Anelva C-7100 thinfilm sputtering system or the like which typically includes threephysical vapor deposition (PVD) chambers each having five targets, anoxidation chamber, and a sputter etching chamber. At least one of thePVD chambers is capable of co-sputtering. Usually, the sputterdeposition process involves an argon sputter gas and the targets aremade of metal or alloys to be deposited on a substrate. The bottomconductor layer 24 and overlying MTJ layers may be formed after a singlepump down of the sputter system to enhance throughput.

In a preferred embodiment, the MTJ stack of layers 31 is fabricated onthe bottom conductor layer 24 by sequentially forming a seed layer 25,AFM layer 26, SyAF pinned layer 27, tunnel barrier layer 28, free layer29, and a capping layer 30. The seed layer 25 may have a thickness ofabout 40 to 60 Angstroms and is preferably a layer of NiCr with athickness of 45 Angstroms and a Cr content of about 35 to 45 atomic %.However, NiFe, NiFeCr, or other suitable materials may be used as theseed layer 25 instead of NiCr. When the seed layer 25 is grown on anamorphous Ta capping layer in the bottom conductor layer 24, a smoothand dense (111) seed layer structure results that promotes smooth anddensely packed growth in subsequently formed MTJ layers.

The AFM layer 26 is preferably made of MnPt with a thickness of about100 to 200 Angstroms and more preferably 150 Angstroms although an IrMnlayer having a thickness from about 50 to 100 Angstroms or a film madeof NiMn, OsMn, RuMn, RhMn, PdMn, RuRhMn, or MnPtPd or the like is alsoacceptable. In the exemplary embodiment, the AFM layer is magneticallyaligned in the y-axis direction. An external magnetic field may beapplied during the deposition of an MTJ layer such as an AFM layer or aferromagnetic (FM) layer to influence a magnetization along a certainaxis.

The SyAF pinned layer 27 has an AP2/coupling layer/AP1 configuration.Use of a SyAF pinned layer in the MTJ structure not only improvesthermal stability but also reduces the interlayer coupling field (offsetfield) applied to the free layer. The AP2 layer is formed on the AFMlayer 26 and is preferably comprised of CoFe with a composition of about25 atomic % Fe and with a thickness of about 20 to 30 Angstroms and morepreferably 23 Angstroms. The magnetic moment of the AP2 layer is pinnedin a direction anti-parallel to the magnetic moment of the AP1 layer. Aslight difference in thickness between the AP2 and AP1 layers produces asmall net magnetic moment for the SyAF pinned layer 26 along the y-axis.Exchange coupling between the AP2 layer and the AP1 layer is facilitatedby a coupling layer that is preferably comprised of Ru with a thicknessof about 7.5 Angstroms although Rh or Ir may be used instead of Ru. In afirst embodiment, the AP1 layer on the Ru coupling layer has a thicknessof about 15 to 25 Angstroms, and more preferably 20 Angstroms, and iscomprised of amorphous CoFeB with a composition of about 40 to 60 atomic% Co, 20 to 40 atomic % Fe, and 15 to 25 atomic % B, and more preferably60 atomic % Co, 20 atomic % Fe, and 20 atomic % B. It is important thatthe B content be at least 15% in order to achieve an amorphous CoFeBlayer. The range for Fe content mentioned above is selected as acompromise between a high Fe content to increase the MR ratio and a lowFe content to maintain a low Hc value and low magnetostriction value.

Above the SyAF pinned layer 27 is formed a thin tunnel barrier layer 28that is preferably MgO. Unlike a method commonly used in the prior artwhere a MgO tunnel barrier is formed by a sputter deposition method, theinventors advantageously employ a procedure where a Mg layer about 8Angstroms thick is deposited followed by an in-situ radical oxidation(ROX) or natural oxidation (NOX), and then deposition of an additionalMg layer about 2 to 6 Angstroms thick. The resulting MgO tunnel barrieris believed to have a thickness greater than 10 Angstroms. Althoughother tunnel barrier layers such as AlOx or AlTiOx may be used insteadof MgO, the performance of the MTJ element will not be as high as when aMgO tunnel barrier is employed. The tunnel barrier layer 28 hasexcellent smoothness and uniformity in part because of the smoothunderlying MTJ layers. The ROX or NOX process is preferably performed inan oxidation chamber within the sputter deposition system. In oneembodiment, the ROX process is comprised of a RF power of about 500Watts and an oxygen flow rate of 0.4 to 0.8 standard liters per minute(slm) and preferably 0.6 slm for a period of about 15 to 50 seconds.Optionally, NOX process conditions comprised of a 1 torr pressure and anoxygen flow rate of from 0.1 to 1 slm and preferably 1 slm for about 60to 120 seconds may be employed to oxidize the Mg layer on the SyAFpinned layer 27.

It has been shown that an MTJ made with a crystalline MgO barrier layerand a CoFeB free layer is capable of delivering a very high dR/R asdescribed in the prior art references. High dR/R is a result of coherenttunneling in which electron symmetry of the ferromagnetic electrode ispreserved in tunneling through the crystalline MgO barrier. Formation ofan MgO tunnel barrier that optimizes MTJ performance with respect to Jc,RA, Rp_cov, and dR/R will be described in a later section.

The free layer 29 formed on the tunnel barrier layer 28 is preferablymade of the same amorphous CoFeB composition as in the AP1 portion ofthe pinned layer 27, and more preferably has a Co₆₀Fe₂₀B₂₀ composition.The free CoFeB free layer 29 has a thickness between 20 and 30 Angstromsand is magnetically aligned along the y-axis (pinned layer direction).When the MTJ is elliptically shaped as seen in a top view (not shown),the easy axis of the MTJ element is along the long axis (y-direction).

Another important feature of the present invention is the capping layer30 that is formed on the free layer 29. In a preferred embodiment, thecapping layer 30 is a composite with a lower layer made of Hf with athickness of from 5 to 50 Angstroms and an upper Ru layer having athickness of from 20 to 100 Angstroms, and preferably 100 Angstroms. Acapping layer with a lower Hf layer that contacts the free layer 29 hasthe effect of reducing the magnetic “dead layer” at the freelayer/capping layer interface according to a mechanism explainedpreviously in a related MagIC Technologies, Corp. patent applicationHMG06-011/012 which is herein incorporated by reference in its entirety.The so-called magnetic dead layer is typically a 3 to 6 Angstrom thickinterface between the free layer and capping layer wherein someintermixing of layers has occurred. For example, in a conventionalNiFe/Ru or NiFe/Ta free layer/capping layer configuration, Ru or Ta maymigrate into a NiFe free layer and thereby reduce the magnetic moment ofthe free layer and dR/R of the MTJ. A magnetic dead layer is indicativeof poor lattice matching and alloying between the free layer andadjoining capping layer. The mechanism for enhancing TMR (dR/R) isbelieved to be based on a capping layer that is highly effective ingettering oxygen from an adjacent free layer. Other elements such as Zrthat have a good oxygen gettering capacity may be used in place of Hf asthe lower layer in the composite capping layer. However, Hf is preferredover Zr as an oxygen gettering agent because it has a higher oxidationpotential. Both Hf and Zr are believed to be better oxygen getteringagents than Ta because they have a lower electronegativity (higheroxidation potential) as listed in the on-line websitehttp://en.wikipedia.org/wiki/Periodic_table.

Hafnium also has a higher oxidation potential than Ni, Fe, and Co andtherefore is very effective in gettering oxygen from an adjacent freelayer that is comprised of one or more of those elements. Previously,the inventors have practiced a process in which a Ru/Ta/Ru trilayerconfiguration was employed as a capping layer. However, the primarygetter agent, Ta, is one layer removed from the free layer in thisconfiguration and a Ru inner layer leads to dR/R degradation.Furthermore, the Bs is typically much higher for a Hf capping layer thanfor a Ta capping layer as shown in a later section. The upper layer inthe capping layer 30 is preferably comprised of Ru to prevent oxidationof the lower Hf layer and to preserve the Hf oxidation potential. Otherdesirable properties of the upper Ru layer are that it ensures goodelectrical contact with an overlying bit line (not shown), is inert tooxidation during annealing, and is a low resistance conductor.

The present invention also encompasses an annealing step after all ofthe MTJ layers have been deposited. For example, in the exemplaryembodiment, the MTJ stack of layers having an MgO tunnel barrier layermay be annealed in a vacuum by applying a magnetic field of 10K Oe inmagnitude along the y-axis (easy axis) for 1 to 5 hours at a temperatureof about 250° C. to 300° C., and preferably 1 hour at 265° C.

After all of the MTJ layers have been deposited and annealing iscompleted, an MTJ element 31 with sidewalls is fabricated by firstcoating and patterning a photoresist layer 32 that has a width w on thecapping layer 30. Next, the photoresist layer 32 is employed as an etchmask during an IBE or Reactive Ion Etch (RIE) sequence that removesregions of the MTJ stack of layers 25-30 which are not protected by theetch mask. Optionally, a hard mask layer (not shown) such as Ta about400 to 600 Angstroms thick may be deposited on the capping layer 30prior to coating the photoresist layer 32. The patterned photoresistlayer 32 serves as an etch mask during a RIE process to removeunprotected regions of the hard mask layer. Then the photoresist layer32 is stripped and the hard mask serves as a mask for a second RIEprocess that etches unprotected regions of layers 25-30. As a result, anMTJ element 31 is formed that typically has sloped sidewalls in whichthe capping layer 30 has a width w and the seed layer 25 has a widthgreater than w.

Referring to FIG. 3, the photoresist layer 32 is removed after theaforementioned IBE or RIE etch sequence by a conventional method thatmay involve a wet stripper or an oxygen ashing process. A standardcleaning step may be performed at this point to ensure that all organicresidue is removed after the stripping step. Then a third insulationlayer 33 is formed on the bottom electrode 24 and adjacent to the MTJsidewalls by a conventional method that may involve depositing aninsulation material with an appropriate dielectric constant and thenplanarizing the third insulation layer 33 to be coplanar with the topsurface 30 a of the MTJ element.

The next step in fabricating the MRAM cell 40 is to form a top conductor(bit line) 34 on the third insulation layer 33 and that contacts the topsurface 30 a of the MTJ element. The bit line 34 is typically aligned ina direction orthogonal to that of the word line 21 and may be comprisedof more than one layer. For instance, a top conductor layer such as Cu,Au, Ru, or Al may be enclosed on the sides and bottom by a diffusionbarrier layer, which is also an adhesion layer, as appreciated by thoseskilled in the art. In the exemplary embodiment, the bit line 34 is usedas a write line to carry current that passes through the MTJ 31 in a CPP(current perpendicular to plane) configuration. The spin-transfer effectarises from the spin-dependent electron transport properties offerromagnetic-insulator-ferromagnetic multilayers. When a spin-polarizedcurrent transverses a magnetic multilayer in a CPP configuration, thespin angular moment of electrons incident on a ferromagnetic layerinteracts with magnetic moments of the ferromagnetic layer near theinterface between the ferromagnetic and insulator layers. Through thisinteraction, the electrons transfer a portion of their angular momentumto the ferromagnetic layer. As a result, a spin-polarized current canswitch the magnetization direction of the ferromagnetic layer if thecurrent density is sufficiently high, and if the dimensions of themultilayer are small. In addition, for spin transfer to be able toswitch the magnetization direction of the free layer 29, theferromagnetic layer (free layer) must be sufficiently thin.

COMPARATIVE EXAMPLE 1

An experiment was conducted to compare the performance of a MTJfabricated according to the first embodiment of the present inventionwith a conventional MTJ based on a CoFeB/MgO/CoFeB pinned layer/tunnelbarrier/free layer configuration described by J. Hayakawa et. al inJapan J. Appl. Phys. V 44, p. L587 (2005). In this experiment, RA istargeted at 50 Ω-μm² and the free layer is made of Co₄₀Fe₄₀B₂₀. The MTJstack is formed on a Ta/Ru200/α-Ta100 bottom electrode and has thefollowing succession of layers in order from bottom to top: 45 AngstromNiCr seed layer; 150 Angstrom MnPt AFM layer;Co₇₅Fe₂₅23/Ru7.5/Co₆₀Fe₂₀B₂₀21 SyAF pinned layer; MgO tunnel barrier; 22Angstrom Co₄₀Fe₄₀B₂₀ free layer; and a Ta/Ru capping layer. The MgOlayer was RF sputter deposited in the Hayakawa method but in thisexperiment was formed by first depositing an 8 Angstrom thick Mg layerfollowed by a ROX process (500 Watts, 0.6 standard liters per minute ofO₂) and then deposition of a 4 Angstrom thick Mg layer. TMR (dR/R) wasfound to be 130% and 189% for the MTJs with 280° C. and 360° C.annealing temperatures, respectively, in this experiment. It isnoteworthy that the dR/R=130% for the 280° C. annealed MTJ is more than2× the value reported by the Hayakawa reference. High dR/R indicatesthat the MgO formed by the Mg8-ROX—Mg4 process of the present invention,when annealed at 280° C., has already achieved good crystal orientationcomparable to the RF-sputtered MgO with >350° C. annealing by Hayakawa.In terms of Bs, it should be noted that the Bs (0.72) of the 280° C.annealed MTJ is less than the Bs (0.78) of the crystalline CoFeB freelayer with 360° C. annealing. The magnetic properties of the films inTable 1 were measured with a CIPT and B-H looper.

TABLE 1 Magnetic Properties of MTJs withCo₆₀Fe₂₀B₂₀/MgO(ROX)/Co₄₀Fe₄₀B₂₀ pinned/tunnel barrier/free layerconfiguration Row ROX Cap Anneal RA MR Bs Hc He Hk 1 50 sec. Ta50/Ru100280° C. 52 120% 0.72 18.26 −2.25 26.4 2 50 sec. Ta50/Ru100 360° C. 54189% 0.78 19.09 −9.56 36.0

COMPARATIVE EXAMPLE 2

As shown previously in equation (2), Jc is scaled with Ms. It is knownthat the Ms of a Co₆₀Fe₂₀B₂₀ free layer is less than that of aCo₄₀Fe₄₀B₂₀ free layer. To further reduce Jc, the MTJ free layer in thisinvention is made of Co₆₀Fe₂₀B₂₀. The magnetic performance properties ofMTJs having a Co₆₀Fe₂₀B₂₀/MgO/Co₆₀Fe₂₀B₂₀ pinned/tunnel barrier/freelayer configuration are shown in Table 2. RA target for the MTJ in thisexperiment is 20 Ω-μm². The other layers with the exception of thecapping layer have the same composition and thickness as in the firstexperiment. In one example (row 2), the 50 Angstrom thick Ta portion ofthe composite capping layer is replaced by a 50 Angstrom thick Hf layer.The upper portion of the capping layer is a 100 Angstrom thick Ru layer.The MgO layer is formed with the same 500 W, 0.6 slm process as was usedto generate the data in Table 1.

TABLE 2 Magnetic Properties of MTJs withCo₆₀Fe₂₀B₂₀/MgO(ROX)/Co₆₀Fe₂₀B₂₀ pinned/tunnel barrier/free layerconfiguration Row ROX Cap Anneal RA MR Bs Hc He Hk 1 20 sec. Ta50/Ru100280 C. 19 105% 0.58 11.45 3.86 31.2 2 20 sec. Hf50/Ru100 280 C. 19 120%0.68 13.35 2.43 35.1 3 20 sec. Ta50/Ru100 360 C. 18 160% 0.63 12.73−15.6 37.3

Comparing row 3 in Table 2 to row 2 in Table 1, the Bs of a Co₆₀Fe₂₀B₂₀free layer is found to be less than the Bs of a Co₄₀Fe₄₀B₂₀ free layeras indicated by the ratio Bs(Co₆₀Fe₂₀B₂₀)/(Co₄₀Fe₄₀B₂₀)=0.63/0.78=0.81.Thus, Ms of a Co₆₀Fe₂₀B₂₀ free layer is about 80% that of a Co₄₀Fe₄₀B₂₀free layer. Consequently, Hc for the Co₆₀Fe₂₀B₂₀ free layer is lowerthan Hc for the Co₄₀Fe₄₀B₂₀ layer. For RA=20 Ω-μm², dR/R of the 360° C.and 280° C. annealed and Ta capped MTJs is 160% and 105%, respectively.Comparing row 1 to row 2 in Table 2, it is interesting to note that for280° C. annealed MTJs, a Hf cap leads to a significantly higher Bs(0.68) than with a Ta cap (0.58). Thus, a Hf cap has the effect ofreducing the magnetic “dead layer” that occurs at the CoFeB freelayer/capping layer interface. Another advantage is that the Hf cappedMTJ has a higher dR/R than for a Ta capped MTJ. When considering theentire MTJ stack, He is related to the roughness of the CoFeB/MgOinterface and the so-called Neel orange peel effect as understood bythose skilled in the art. As shown in Table 2, He for the 280° C.annealed MTJ is lower (in absolute value) than for a 360° C. annealedMTJ which means the 280° C. MTJ has a smoother pinned layer/tunnelbarrier interface. The smooth Co₆₀Fe₂₀B₂₀/MgO interface tends to confirmthat the MgO tunnel barrier fabricated according to the presentinvention is formed on top of an amorphous CoFeB AP1 pinned layer. Thus,it follows that the Co₆₀Fe₂₀B₂₀ free layer in a 280° C. annealed MTJ isalso amorphous. During a writing operation to achieve a symmetrical R-Iresponse, for example Ic⁺˜Ic⁻, a much lower offset field H_(off) (Ha) isnecessary for the 280° C. annealed MTJ compared with a 360° C. annealedMTJ which is yet another advantage provided by the preferred embodimentof the present invention that comprises a low annealing temperature suchas 280° C.

Another piece of information relevant to the present invention isprovided by C. Bilzer et. al in “Study of the dynamic magneticproperties of soft CoFeB films”, J. Appl. Phys., V 100, p. 053903 (2006)where a MTJ with an amorphous free layer was shown to have a lowerdamping constant α than a MTJ with a crystalline free layer. Thus, anadditional advantage afforded by the MTJ configuration of the presentinvention is a lower Jc than MTJs based on crystalline CoFeB layersbecause of equation (2) where Jc is proportional to “a”. As the dampingconstant decreases, the switching current also decreases which isdesirable for Spin-RAM applications.

With respect to Rp_cov, a comparison in MRAM technology has been madebetween a CoFeB/AlOx/NiFe MTJ and a CoFeB/MgO/NiFe MTJ that are bothannealed at 280° C. In the former, AlOx is amorphous, and in the lattercase, MgO is crystalline as is the overlying NiFe free layer. It hasbeen found that in a MRAM device, the Rp_cov for a CoFeB/MgO/NiFe MTJ inis 2× greater than for a CoFeB/AlOx/NiFe MTJ. In the Sony Spin-RAMexample mentioned previously, CoFeB/MgO/CoFeB has acrystalline/crystalline/crystalline structure that results in aRp_cov=4%. It is very likely that a Rp_cov much less than 4% can beachieved for a Spin-RAM made of a CoFeB (amorphous)/MgO(crystalline)/CoFeB (amorphous) MTJ as in the present invention.Although a TMR/Rp_cov ratio has not been determined as yet for the MTJstructure as disclosed in the first embodiment, it is expected that theRp_cov value will be 3% or less and the resulting TMR/Rp_cov=120%/3%=40will achieve the desired spin-transfer requirement as stated earlier.

When compared to a conventional Ta capped MTJ, a Hf capped MTJ accordingto the present invention in which the capping layer has a Hf/Ruconfiguration has the advantages of higher output signal (dR/R), smalleroffset field (Hoff), and a reduced magnetic “dead layer”. Furthermore,in terms of the thermal stability parameter (K_(u)V/K_(b)T), a Hf cappedMTJ accordingly has an advantage over a Ta capped MTJ.

COMPARATIVE EXAMPLE 3

Another experiment was performed to determine the effect of employing aNOX process rather than a ROX process in forming the MgO tunnel barrierlayer. Otherwise, the composition and thickness of the various layers inthe NOX-MTJ are the same as in Table 2. In rows 2-4 of Table 3, the MgOlayer is formed by a natural oxidation (NOX) process wherein an 8Angstrom thick Mg layer is laid down on the CoFeB pinned layer followedby the NOX step and then a second Mg layer having a thickness of 4Angstroms is deposited. The NOX process comprises exposure of an 8Angstrom thick Mg layer to a 1 torr atmosphere by using an O₂ flow rateof 1 standard liter per minute (slm) for a period of 60 to 100 seconds.

TABLE 3 Magnetic Properties of MTJs with Co₆₀Fe₂₀B₂₀/MgO ROX(NOX)/Co_(6o)Fe₂₀B₂₀ pinned/tunnel barrier/free layer configuration RowROX(NOX) Cap Anneal RA MR Bs Hc He Hk 1 ROX 15 sec. Ta50/Ru100 280 C. 11 96% 0.59 10.96 8.55 34.2 2 NOX 60 sec. Ta50/Ru100 280 C. 9.7 103% 0.6110.20 3.73 37.53 3 NOX 60 sec. Hf50/Ru100 280 C. 9.8 110% 0.69 11.333.26 43.21 4 NOX 80 sec. Ta50/Ru100 360 C. 12 164% 0.64 12.73 −21.0 40.6

Referring to row 1 in Table 3, when ROX cycle time is reduced from 20seconds to 15 seconds, a RA target of 10 Ω-μm² is obtained. However, thedR/R is lowered to 96%. The magnetic performance of NOX-MTJs formedaccording to a first embodiment of the present invention is listed inrows 2 to 4. Comparing row 1 to row 2, it is obvious that He is muchlower for the NOX-MTJ than for the ROX-MTJ. For a 280° C. annealedNOX-MTJ, a dR/R above 100% is achieved. Again, a Hf capped MTJ deliversa higher dR/R and lower He (row 3 vs. row 2) than a Ta capped MTJ.Therefore, for a lower RA of about 10 Ω-μm², it is preferable to form aMgO tunnel barrier by employing a NOX process according to the presentinvention as well as incorporating amorphous CoFeB pinned and freelayers, and Hf as the bottom layer in the capping layer.

In addition to a dR/R>100%, a low RA value and low He, the firstembodiment of the present invention also provides a Jc (critical currentdensity) of less than 1×10⁶ A/cm², a substantial improvement over priorart examples where magnetization switching due to a spin-transfer effectis >2×10⁶ A/cm², too high to be useful for Spin-RAM applications. A MTJhaving an [amorphous CoFeB/MgO (ROX or NOX)/amorphous CoFeB] AP1 pinnedlayer/tunnel barrier/free layer configuration with a capping layer madeof a lower Hf layer and an upper Ru layer substantially minimizes thesize of the “dead layer” between the free layer and capping layerthereby enabling a higher dR/R to be realized. However, in a readoperation where the MTJ is 300 mV biased, TMR is lowered to around 60%.As a result, TMR/(Rp_cov=4%)=15 which is slightly below the desiredvalue of 20 for a Spin-RAM. Thus, further improvement in TMR (dR/R) forthe MTJ according to the objectives set forth in the present inventionis desirable.

In a second embodiment, the same elements are retained from the firstembodiment except that the AP1 portion of the SyAF pinned layer 27 ismodified and has a composite configuration wherein an amorphous CoFeBlower AP1 layer is formed adjacent to the coupling layer (Ru, Rh, Ir)and a crystalline CoFe upper AP1 layer is formed adjacent to the MgOtunnel barrier. The amorphous CoFeB lower AP1 layer has the samecomposition as described previously but the thickness is reduced to arange of 12 to 18 and preferably 15 Angstroms. The crystalline CoFeupper AP1 layer has a Fe content between 20 and 40 atomic %, andpreferably 25 atomic %, and a thickness between 5 and 7 Angstroms, andpreferably 6 Angstroms. Thus, in a preferred embodiment, the AP1 layeris comprised of a lower Co₆₀Fe₂₀B₂₀ layer and an upper Co₇₅Fe₂₅ layer.The capping layer 30 may be made of Hf, Zr, or Ta. The same process flowis followed to form the MTJ element as described in the first embodimentwith the exception of the deposition of an additional Co₇₅Fe₂₅ layer asthe upper layer in the AP1 composite stack. The annealing process forthe MTJ stack of layers preferably comprises applying a 10000 Oe fieldfor 1 to 2 hours at about 265° C.

COMPARATIVE EXAMPLE 4

An experiment was performed to compare a MTJ made according to thesecond embodiment of the present invention with an MTJ similar to thatdescribed in the Hosomi and Hayakawa prior art references shown in row 1of Table 4. The composition and thickness of the various layers in theROX-MTJ are the same as in Table 2 with the exception of rows 3-4 wherea composite AP1 layer formed according to the present invention isemployed. The AP2/coupling layer portion of the pinned layer (Co₇₅Fe₂₅)23/Ru 7.5 is the same as in earlier examples. The ROX process used foreach row in Table 4 is 500 W and 0.6 slm O₂ flow rate for 20 seconds.

TABLE 4 Magnetic Properties of MTJs with AP1/MgO ROX/Co₆₀Fe₂₀B₂₀ 20pinned/tunnel barrier/free layer configuration Row AP1 Cap Anneal RA MRBs Hc He Hk 1 Co₆₀Fe₂₀B₂₀ 21 Ta50/Ru100 360 C. 12 160% 0.60 12.7 −21.040.6 2 Co₆₀Fe₂₀B₂₀ 21 Ta50/Ru100 265 C. 16  72% 0.49 8.9 10.2 34.4 3Co₆₀Fe₂₀B₂₀ 15/ Ta50/Ru100 265 C. 11 123% 0.51 8.8 3.41 31.2 Co₇₅Fe₂₅ 64 Co₆₀Fe₂₀B₂₀ 15/ Hf50/Ru100 265 C. 11 133% 0.61 11.3 3.21 36.8 Co₇₅Fe₂₅6

The prior art MTJ listed in row 1 in Table 4 has fully crystallineCo₆₀Fe₂₀B₂₀ AP1 and Co₆₀Fe₂₀B₂₀ free layers because of the 360° C. (2hrs, 10000 Oe) annealing process. The dR/R=160% is similar to the valuescited for this type of MTJ in the prior art that have a 350° C.annealing temperature. Note that the Bs of the free layer is 0.60 nwwhile the Bs for the MTJ stack in row 2 which was annealed for 2 hoursat 265° C. with a 10000 Oe applied field is 0.49 nw for a ratio of0.60/0.49=1.22. Again, this result is comparable to the ratio obtainedin the Hayakawa reference for Bs [350° C. annealed MTJ stack (CoFeB freelayer)]/Bs [270° C. annealed MTJ stack]=1.6T/1.3T=1.23. C. Park et. alin “Annealing effects on structure and transport properties ofrf-sputtered CoFeB/MgO/CoFeB magnetic tunnel junction”, J. Appl. Phys. V99, 08A901 (2006), shows that the Co₆₀Fe₂₀B₂₀ free layer after 270° C.annealing is amorphous while a Co₆₀Fe₂₀B₂₀ free layer after 360° C.annealing is crystalline. It follows that the Co₆₀Fe₂₀B₂₀ AP1 andCo₆₀Fe₂₀B₂₀ free layer of the present invention will also remainamorphous after a 265° C. annealing process. Referring to row 2 of Table4, dR/R=72% for the 265° C. annealed MTJ is higher than dR/R reported inthe Hayakawa reference. However, dR/R=72% is still lower than dR/R=96%for the same MTJ stack made with 280° C. annealing. This discrepancy maybe explained by the Park reference which postulates that for 280° C.annealing, the amorphous Co₆₀Fe₂₀B₂₀ free layer is locally crystallizedat the MgO/CoFeB interface.

Another advantage of the present invention is that MgO tunnel barrierlayers in the prior art such as in the Sony and Hayakawa references areformed by RF sputtering from a dielectric target hereafter referred toas Sput-MgO whereas the MgO tunnel barrier formed herein is made by aROX or NOX process of a Mg deposited layer as described earlier. Ahigher dR/R demonstrated by the MTJ in row 2 than the dR/R=49% in theprior art reference means that a MgO layer having a highly oriented(100) crystal structure is more favorably formed in the MTJs of thepresent invention than by a Sput-MgO process.

Since better MgO orientation yields higher dR/R for an MTJ, we attemptedto achieve even higher dR/R values than in the first embodiment byoptimizing the AP1 “substrate” on which the tunnel barrier layer isformed. During our investigation, we discovered that a composite AP1layer wherein a lower amorphous Co₆₀Fe₂₀B₂₀ layer having a smoothsurface and an upper Co₇₅Fe₂₅ layer with a bcc crystal structureprovides further improvement in the dR/R ratio of a MTJ comprised of acrystalline (100) MgO (ROX/NOX) layer and an amorphous CoFeB free layer.In row 3 of Table 4, a dR/R>120% is achieved with a Ta capping layer andwith a 265° C. annealing temperature. The dR/R increases to >130% underthe same process conditions when a Hf capping layer is employed and Bsfor this MTJ in row 4 is equivalent to that of a 360° C. annealed freelayer. Another improvement when switching to the composite AP1 layer ofthe second embodiment is that the RA is reduced from 16 ohm-μm² (row 2)to 11 ohm-μm² in row 3. Moreover, the interlayer coupling field (He)between the pinned layer and free layer is considerably decreased andthereby reduces the Ha (H_(off)) term in equation (1). It should also bepointed out that the MgO layer formed herein has a thickness of about 10Angstroms which is greater than the Sput-MgO generated tunnel barriersin prior art. As a result, the V_(dB) (dielectric breakdown voltage) forthe thicker ROX—MgO layer of the present invention is measured tobe >1.2 volts, substantially higher than that of the prior art.

COMPARATIVE EXAMPLE 5

Another experiment was conducted to evaluate MTJs in which the MgOtunnel barrier layer is formed by a NOX process as described previouslywith regard to Table 3 and Comparative Example 3. The MgO layer isprepared by depositing an 8 Angstrom thick Mg layer on the AP1 layerfollowed by a NOX process comprised of an O₂ flow rate of 1 slm and at 1torr pressure for about 80 to 300 seconds. Otherwise, the compositionand thickness of the various layers in the NOX-MTJs in rows 1-4 of Table5 are the same as in the corresponding rows in Table 4 with theexception of row 1 where the free layer thickness is increased from 20to 22 Angstroms due to interdiffusion between the free layer and Tacapping layer.

TABLE 5 Magnetic Properties of MTJs with AP1/MgO NOX/Co₆₀Fe₂₀B₂₀ 20pinned/tunnel barrier/free layer configuration Row NOX AP1 Cap Anneal RAMR Bs Hc He Hk 1  80″ Co₆₀Fe₂₀B₂₀ 21 Ta50/Ru100 360 C. 9 166% 0.64 11.5−18.7 40.3 2  80″ Co₆₀Fe₂₀ B₂₀ 21 Ta50/Ru100 265 C. 10  81% 0.49 9.636.45 33.6 3 300″ Co₆₀Fe₂₀B₂₀ 15/ Ta50/Ru100 265 C. 6 130% 0.51 9.79 2.7331.5 Co₇₅Fe₂₅ 6 4 300″ Co₆₀Fe₂₀B₂₀ 15/ Hf20/Ru100 265 C. 6 143% 0.6312.5 1.68 36.1 Co₇₅Fe₂₅ 6

Compared to the ROX—MgO MTJs in Table 4, the MTJs fabricated with aNOX—MgO layer yield a higher dR/R and a lower RA value of ≦10 ohm-μm².Thus, the NOX—MgO MTJs according to the present invention not onlyresult in a smaller RA but a smaller He. In the row 4 example with a Hfcapping layer, the dR/R is as high as 143% which is almost 3× greaterthan the result reported in the Hayakawa reference.

With regard to Rp_cov, a comparison can be made in a 1-Mbit MRAM for theCoFeB/AlOx/NiFe MTJ and the CoFeB/MgO/NiFe MTJ. In the former, AlOx isamorphous and in the latter, MgO is crystalline along with the NiFe freelayer. The Rp_cov for the former with both a crystalline (MgO) tunnelbarrier and a crystalline NiFe free layer is 2× that of the MTJs with anamorphous (AlOx) tunnel barrier and a crystalline NiFe free layer. InSony's Spin-RAM reference cited previously, MgO/CoFeB has a(crystalline/crystalline) interface structure to give a Rp_cov=4%.Considering the reduction in Rp_cov by employing a(crystalline/amorphous) interface structure in the 1-Mbit MRAMcomparison above, we believe that the Rp_cov of the present inventionthat has a crystalline/amorphous interface and a MgO/CoFeB tunnelbarrier/free layer configuration should be substantially reduced below4%.

Spin-RAM has two reliability issues. One is thermal instability thatdepends on free layer volume (V) and the second is the lifetime of thetunneling oxide that depends on the thickness of the tunnel barrier. Ina preferred embodiment of the present invention where the capping layeris comprised of Hf, the free layer has in effect a greater thickness dueto a thinning of the “dead layer” than when a capping layer with a loweroxidation potential is used. In other words, the volume (V) of the freelayer is increased and thermal stability improves since V is related tothe thermal stability function term K_(u)V/k_(b)T. Therefore, the MTJstructure of the present invention has the beneficial effect of improvedstability based on a reduction in dead layer volume to increase theeffective volume of free layer at the same physical thickness, andbecause of a thicker MgO tunnel barrier which improves the tunnelingoxide lifetime. With a dR/R of ≧130% and a Rp_cov believed to besubstantially less than 4% as provided by an MTJ according to the secondembodiment of the present invention, a Jc value reduction to <1×10⁶A/cm² is anticipated.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

1. An MTJ element for reducing spin-transfer magnetization switchingcurrent in a magnetic device, comprising: a pinned layer having anAP2/coupling layer/AP1 configuration wherein the AP2 layer is formed onan AFM layer and the AP1 layer is a composite made of a lower amorphousCoFeB layer and an upper crystalline CoFe layer; a crystalline MgOtunnel barrier formed on the upper crystalline CoFe AP1 pinned layer; anamorphous CoFeB free layer formed on the MgO tunnel barrier, both ofsaid amorphous lower AP1 CoFeB layer and amorphous CoFeB free layer havean identical composition represented by Co_(x)Fe_(y)B_(z) where x isfrom about 40 to 60 atomic %, y is from about 20 to 40 atomic %, and zis from about 15 to 25 atomic %; and a composite capping layer formed onthe CoFeB free layer wherein the composite capping layer is comprised ofa lower layer that contacts the free layer and an upper layer, and saidlower layer is made of a metal having an oxidation potential greaterthan that of Co, Fe, and Ta to provide a high oxygen getteringcapability.
 2. The MTJ element of claim 1 wherein the magnetic device isa Spin-RAM.
 3. The MTJ element of claim 1 wherein the lower amorphousCoFeB AP1 pinned layer and amorphous CoFeB free layer have a Co₆₀Fe₂₀B₂₀composition.
 4. The MTJ element of claim 1 wherein the lower layer inthe composite capping layer is comprised of Hf or Zr.
 5. The MTJ elementof claim 4 wherein the composite capping layer has a Hf/Ru configurationin which the lower Hf layer has a thickness from about 5 to 50 Angstromsand the upper Ru layer has a thickness from about 20 to 100 Angstroms.6. The MTJ element of claim 1 wherein the AP2 portion of the pinnedlayer is comprised of CoFe and the coupling layer is Ru.
 7. The MTJelement of claim 1 wherein the upper crystalline CoFe AP1 pinned layerhas a Co₇₅Fe₂₅ composition.